Solar collectors utilize incident solar radiation by absorbing visible light and converting this electromagnetic energy to heat. The heat is generally released to a heat-storing, free-flowing medium and supplied to a storage reservoir by the flow. Solar collector surfaces consist typically of coated copper or aluminum sheets which are welded to pipelines in order to ensure good heat transfer into the storage medium conducted within the tubes.
The ultimately achievable useful heat depends significantly on the absorption capacity of the collector coating, which must ideally be high for light over the entire solar spectrum. However, the heat thus obtained must at the same time not be emitted again in the infrared spectrum to an excessive degree. The coating is therefore additionally required to have a low emission capacity in the wavelength range above about 2000 nanometers, which is associated with a high reflectivity in this spectral region.
Absorber coatings which satisfy the stated demands are already commercially available and are referred to as selective. Large-area production is possible in highly optimized coating plants, for instance by means of physical vapor deposition (VPD) for example onto continuous copper ribbons, which allows coated areas of several square meters per hour to be achieved. Additionally known are sputtering processes and also a combined CVD/sputtering process. However, these are high-vacuum processes with a considerable level of apparatus complexity. For further information, reference is made at this point to the publication BINE projektinfo 5/99 from the BINE information service published by the Fachinformationszentrum Karlsruhe.
As is well known, robust, firmly adhering coatings with long-term thermal and chemical stability can also be obtained by sol-gel coating methods. The advantages of such methods, as well as the low demands on equipment and process control, lie in the high variability of the usable materials with regard to composition and layer structure, in the means of coating virtually any nonplanar surfaces, in the relatively low energy demand and not least in the combinability of different coating steps to obtain multifunctional layers, as can be achieved with other known processes with a barely acceptable level of cost and inconvenience, if at all. From that point of view, it seems astonishing that so little attention has been paid to date to the production of solar absorber layers by means of sol-gel methods.
The sole currently know exception and hence, at the same time, closest prior art is the patent DE-C2 101 21 812 which protects a process for producing a selective absorber layer by dip-coating. To this end, a titanium-containing oxide layer, which is said to contain so-called selectively absorbing structural elements, is formed on a magnesium-containing aluminum sheet. Unfortunately, though, it is not clearly evident from the publication what the nature or what the origin of these “structural elements” is supposed to be. This leads at first to the suspicion that specifically a titanium dioxide coating of the substrate as the absorber might be what is meant. However, this would be clearly contradicted by experimental findings (see also FIG. 1). It therefore remains to be assumed that the specific reference to the Mg—Al substrate is essential to the invention here and an oxidation of the substrate surface itself—perhaps a result of the additions of nitric acid?—is brought about in order to achieve the absorber effect described. If this were to be the case, the method here would not be a conventional sol-gel coating but a chemical surface treatment. In any case, the teaching of DE-C2 101 21 812 does not give any reason to believe that the coating described there might be a solar absorber coating for a plurality of—or even virtually any—substrates.
It is thus an object of the invention to specify a process for producing an absorber coating favorable for solar heating, by which the layers can be formed with known sol-gel coating methods (spraying, dipping, spinning) on various substrates, especially on copper, aluminum, stainless steel or glass.